1. Field of the Invention
The present invention relates to a non-contact voltage probe apparatus for searching for an electromagnetic disturbing source.
A method of specifying an electromagnetic interference invading route, which is proposed by the present invention, can be used to solve EMC (Electromagnetic Compatibility) problems that arise in electronic equipment, such as in telecommunication equipment, an information processing apparatus, etc. The utilization of the method according to the present invention makes it possible to specify the invading route with which the electromagnetic interference invades the equipment. As a result, effective countermeasures can be taken against the electromagnetic interference, and the electromagnetic interference source can be specified. Thus, the present method makes it useful to solve EMC problems. Further, because the determination of the interference invading route, which has heretofore depended on the experience and particular knowledge of the problem, can be objectively performed according to a physical quantity, even inexperienced users can easily determine the interference invading route.
2. Description of the Prior Art
With advances in semiconductor technology, information telecommunication equipment is now advancing in the areas of high density, high integration, and low-voltage driving. Further, the number of connecting cables for providing electrical connections between a plurality of pieces of equipment has increased, and the configuration of the connections between the equipment has become complicated. Therefore, a phenomenon has occurred in which common mode electromagnetic interference induced between the cable connected to the equipment and the earth or ground or the like propagates through the cables and invades the equipment, thereby causing failures in the equipment. With the advanced large scale system, in particular, the public is inconvenienced when the equipment produces this type of trouble, which also tends to occur with the advanced low-voltage driving. The electromagnetic interference that propagates through the cables or the like, is called "conducted interference." The prevention of equipment failure due to the conducted interference has become a significant problem.
According to the "IEEE Standard Dictionary of Electrical and Electronics Terms," the term common mode interference is defined as "Interference that appears between both signal leads and a common reference plane (ground) and causes the potential of both sides of the propagation path to be changed simultaneously and by the same amount relative to the common reference plane (ground)." The term common mode voltage is the mean of the phasor voltages appearing between respective conductors and a specified reference (usually, ground or earth). In contrast, the term differential mode interference is defined as "Interference that causes the potential of one side of the signal propagation path to be changed relative to the other side." The term differential mode voltage is the voltage between any two of a specified set of active conductors.
As part of the countermeasures to be taken against such conducted interference, an apparatus is required which measures the voltage and current of the interference entered therein and accurately recognizes an interference invading route and an interference level. By specifying an electromagnetic interference source as a cause of the equipment failure, an effective countermeasure can be taken, and the cause of the failure can be removed.
The measurement of the voltage and current of the interference under service and operational conditions is necessary to grasp the situation with regard to the malfunctions due to the interference.
It is therefore necessary to use a voltage probe capable of efficiently measuring conducted interference propagated through the cable, and, in particular, measuring a common mode voltage developed between the cable and the ground. Further, the voltage probe should measure the common mode voltage with ease, with satisfactory accuracy, and without the influence of the interference on communication signals under service conditions.
As one method of measuring the voltage, a non-contact voltage probe using capacitive coupling to the cable has been discussed. With capacitive coupling, however, the sensitivity of the probe could be unstable depending on the internal position of the cable and the capacitance between the cable and a surrounding metal body. In particular, the stray capacitance occurring between the cable and the surrounding metal body varies according to surrounding conditions. Therefore, the sensitivity of the probe greatly varies, and the probe is susceptible to the potential of the surrounding metal body.
FIG. 17A shows a conventional non-contact voltage probe apparatus. In the drawing, reference numerals 201A, 202, 203 and 204 respectively indicate a cylindrical electrode, a jig for fixing a cable 220, a high input impedance voltage probe, and a level meter. As shown in FIG. 17A, the cylindrical electrode 201A produces capacitive coupling between a metal body 230, such as a grounded metal cabinet 230, and a cable 221 or the like. In FIG. 17B, an equivalent circuit free of ambient influences is shown. A voltage V.sub.p output from the probe 203 is given by the following equation (1): EQU V.sub.p =j.omega.CR.sub.p /{1+j.omega.R.sub.p (C+C.sub.p)}.times.V(1)
where V, C, R.sub.p and C.sub.p respectively indicate the voltage induced between the cable 220 and ground, the capacitance between the cable 220 and the cylindrical electrode 201A, the input resistance of a the high input impedance voltage probe 203, and the input capacitance of the high input impedance voltage probe 203.
Now, assume that C.sub.q is the capacitance between the reinforcing bars for a building or a grounded metal cabinet 230 or the like and the cylindrical electrode 201A. Also, assume that a voltage V.sub.x exists in another cable 221 or the like, and that a capacitance C.sub.x couples the cable 221 and the cylindrical electrode 201A. In FIG. 17C, an equivalent circuit is shown. As can be seen from this equivalent circuit, the sensitivity of the probe varies according to the capacitances C.sub.x and C.sub.x, and the influence of the voltage V.sub.x is included in a voltage V.sub.p ' Thus, a large error is caused when measuring the voltage V.
The non-contact voltage probe apparatus of the prior art has the following problems in terms of the above description, which result in an inability to accurately reproduce voltage measurements.
(1) According to the ambient conditions of the electrode 201A, the values C.sub.x and C.sub.q of the stray capacitances vary, and the sensitivity of the non-contact voltage probe apparatus varies.
(2) The non-contact voltage probe apparatus is susceptible to the voltage developed in the surrounding cable 221.
Moreover, measuring current using a current probe has previously been used to measure conducted interference propagating through a cable to invade equipment. However, the decision as to where the interference invades greatly depends on the engineer's experiences. It is therefore difficult to accurately specify an interference invading route. Even when resonances occurs, an attempt to specify the invading route by comparing the magnitudes of currents, for example, may be incorrect because the value of the current flowing into the position where the interference invades is not always maximum. For this reason, the invading route cannot be determined from the result of this comparison. Further, the determination of the direction of the current flow, other than d.c. current flow, also has difficulties, and the propagation direction of the electromagnetic interference cannot be specified in this manner either. Therefore, the decision as to the invading route depends on a measurer's experiences, and hence the invading route is often misjudged. As such, the invading route cannot be specified accurately, and thus it is difficult to determine the interference source causing the equipment failure.
Alternatively, a method for determining the interference invading route by connecting and disconnecting a cable connected to the equipment has also been proposed. Since, however, deactivation of the equipment and cutting or the like of the cable are necessary, the influence of interference under actual conditions cannot be judged with accuracy.
Further, when a plurality of interferences have invaded, searching for the interference source by connecting and disconnecting pieces of the cable likewise can not be achieved under actual conditions.